With the breakthrough of the manufacturing technique of the light-emitting diode (LED) in recent years, the luminance and illuminating efficiency of the light-emitting diode are greatly enhanced. Therefore, the light-emitting diode has replaced the fluorescent lamps as the illuminating elements of the next generation. Also, the light-emitting diode has been widely used as a car illuminating device, handheld illuminating device, the backlight source of liquid crystal display (LCD) panels, the traffic signs, and billboards.
It is generally required to drive a plurality of light-emitting diode arrays to provide sufficient light source in LED-related applications. As each light-emitting diode has different characteristics with each other, the currents flowing through the light-emitting diode arrays are unequal with each other. Thus, the luminance of the electronic device employing light-emitting diodes, such as a LCD panel, is not uniform. This would shorten the life of the light-emitting diodes and damage the electronic device.
To address the problem as a result of the unbalanced currents flowing through the light-emitting diode arrays, various current balancing technique for light-emitting diode has been proposed to address this problem. FIG. 1 is a block diagram showing the structure of a conventional multi-channel LED driver circuit. As shown in FIG. 1, the conventional multi-channel LED driver circuit 1 is used to drive a plurality of LED arrays G1-G4. The multi-channel LED driver circuit 1 includes a transformer Tr, a pulse-width modulation (PWM) controller 11, a main switch 12, an output rectifier and filter 13, and a plurality of regulating circuits 14-15. The primary winding Np of the transformer Tr is connected to the main switch 12, and the secondary windings Ns1-Ns4 of the transformer Tr are connected to the output rectifier and filter 13 and the regulating circuits 14-15. The pulse-width modulation (PWM) controller 11 is connected between the control terminal of the main switch 12 and the output rectifier and filter 13.
In operation, the energy of the input voltage Vin is transmitted to the primary winding Np through the main switch 12 by the switching operations of the main switch 12. Thus, each secondary winding Ns1 -Ns4 can generate a secondary voltage and provide the secondary voltage for the output rectifier and filter 13 and the regulating circuits 14-15. The currents provided for the light-emitting diode arrays are balanced by the operations of the output rectifier and filter 13 and the regulating circuits 14-15. Therefore, the current-equaling performance is attained. In order to allow each secondary winding Ns1-Ns4 to provide sufficient power for the output rectifier and filter 13 and the regulating circuits 14-15, the pulse-width modulation (PWM) controller 11 can regulate the duty ratio of the main switch 12 according to the output current of the output rectifier and filter 13.
It can be known from the above descriptions that the conventional multi-channel light-emitting diode (LED) driver requires a transformer with a plurality of secondary windings and a complex structure. Thus, the conventional multi-channel light-emitting diode (LED) driver is costly and bulky. Also, as the output rectifier and filter and the regulating circuits are independent from each other, the pulse-width modulation (PWM) controller can regulate the duty ratio of the main switch according to the output current of the output rectifier and filter 13. However, in order to allow each secondary winding Ns1-Ns4 to provide sufficient power for the output rectifier and filter 13 and the regulating circuits 14-15, the regulation of the duty ratio can not be optimized. Thus, the power provided by the secondary winding for the regulating circuit will be excessive, which indicates that the secondary voltage provided by secondary winding has a larger duty ratio. This would cause a considerable power loss to the regulating circuit and the multi-channel light-emitting diode (LED) driver and deteriorate the operating efficiency of the multi-channel light-emitting diode (LED) driver.
It is incline to develop a multi-channel light-emitting diode (LED) driver to address the aforementioned problems encountered by the prior art.